**1. Introduction**

The ocean accounts for about 71% of the Earth's surface area and is the largest potential resource base on the Earth that has not been fully recognized and utilized by humans. There are extremely rich biological and mineral resources in the ocean. The deep sea is the lowest layer in the ocean, existing below the thermocline, at a depth of 1800 m or more [1]. Deep sea areas with a depth of more than 2000 m account for 84% of the ocean area. Therefore, the surface of the earth is mostly deep sea. The hypsometric profile of the ocean is shown in **Figure 1**.

Scientific research, resource development, engineering construction, and military activities around the ocean usually require accurate acquisition of seafloor topographical information in the area of interest as the basis for data and support. Therefore, understanding ocean topographical information, mapping ocean

#### **Figure 1.**

*Hypsometric profile of the ocean.*

topographical and geomorphological information effectively, and how to obtain ocean topographical information map have become important issues in the current development of marine resources and marine space utilization [2]. In particular, marine topographical information is of immense value in marine space utilization.

Topography involves the recording of terrain, the three-dimensional quality of the surface, and the identification of specific landforms. It is often considered to include the graphic representation of the landform on a map by a variety of techniques, including contour lines, hypsometric tints, and relief shading [3]. Geomorphology is the branch of science that studies the characteristics and configuration and evolution of rocks and landforms [4]. In the ocean, seafloor topography is measured by multibeam echosounder (MBE), and seafloor geomorphology is measured by side-scan sonar (SSS).

Deep sea topographical exploration mainly includes full sea depth topographical detection and near-seafloor micro-topographical detection. The advantage of full sea depth topographical is large spatial range and rapid data acquisition, and the disadvantage is limited accuracy. In contrast, near-seafloor micro-topographical exploration provides accurate detection of the seafloor using MBE, SSS, and bathymetric side-scan sonar (BSSS) carried on-board various underwater vehicles, including deep tow (DT) [5], autonomous underwater vehicle (AUV) [6], remotely operated vehicle (ROV) [7], and human occupied vehicle (HOV) [8–10]. It can obtain more accurate micro-topography and micro-geomorphology of the seafloor compared to full sea depth topographical detection.

In this chapter, the basic principles of three types of near-seafloor micro-topographical mapping sonars are analyzed. Then, four types of underwater vehicles suitable for near-seafloor micro-topographical mapping are briefly discussed. Next, factors affecting mapping and detection results are presented using the Jiaolong HOV and its BSSS as an example. Finally, the entire data processing and mapping methods are presented.

## **2. Near-seafloor micro-topographical mapping sonars**

Three types of seafloor mapping sonars, which can be mounted of deep sea vehicles, are the multibeam echosounder (MBE), and the side-scan sonar (SSS) and the bathymetric side-scan sonar (BSSS).

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**Figure 2.**

*Basic principle of MBE.*

*Advanced Mapping of the Seafloor Using Sea Vehicle Mounted Sounding Technologies*

An MBE works by transmitting a wide-sector-covered sound wave to the seafloor using a transmitting transducer array, and the narrow-beam receives the sound wave using a receiving transducer array. The footprints of the seafloor topography are formed by the orthogonality of the transmission and reception sectors, and these footprints are properly processed. A ping can indicate the water depth values of hundreds or even more seafloor measured points in the vertical plane perpendicular to the heading. Therefore, it is possible to accurately and quickly measure the size, shape, and height variation of underwater targets within a certain width of the route, and to reliably depict the three-dimensional features of the seafloor topography. The basic principle of an MBE is shown in

The beamforming method of MBE can be divided into two types: beam steering method (measuring the round-trip time of the reflected signal at a specific angle) and coherent method (measuring the angle of the reflected echo signal at a specific time). There are two main variables to be measured in an MBE, namely the slant distance or the distance from the acoustic transducer to each point on the seafloor and the angle from the transducer to the bottom of the ocean. All MBEs use one or both beamforming methods to determine these variables. At present, MBE manufacturers using beam steering method include Reson, Kongsberg, ATLAS, L3, and R2Sonic, whereas manufacturers using coherent method include Teledyne Benthos

For large-area exploration of seafloor topography, shipborne deep-water MBE can be used to obtain relatively accurate seafloor topographical data. The frequency of deep-water MBE is generally approximately 12 kHz. A typical beam width is 1°× 1°, and the corresponding beam footprint is 1.75% water depth. For example, when the water depth is 5000 m, the beam footprint is approximately 87.5 m. It can be observed that the shipborne deep-water MBE cannot obtain high-precision seafloor

Underwater vehicles, such as DT, AUV, ROV, and HOV, can carry more highfrequency MBE to near-seafloor to achieve accurate topographical detection. The corresponding MBE is designed with a special pressure-resistant design with a pressure depth of up to 6000 m or even deeper. At present, some commercial

*DOI: http://dx.doi.org/10.5772/intechopen.83448*

**2.1 Multibeam echosounder**

**Figure 2**.

and Geoacoustics.

topographical data.
